This invention relates in general to frequency multiplier stages such as those commonly found in frequency modulated (FM) transmitters. Specifically, the invention provides a tunable coupling network that presents a passaband for a desired harmonic of a fundamental frequency while providing attenuation for undesired harmonics and/or the fundamental frequency itself.
FM transmitters often include frequency multiplier stages which comprise a class C amplifier that is terminated or coupled to a subsequent stage by a filter network. This filter network is tuned so as to pass a desired harmonic of an excitation signal and to reject certain undesired harmonics other than the desired one. For example, in a doubler circuit, the filter network is usually designed to pass a second harmonic and to reject both the fundamental frequency and the third harmonic. Higher order harmonics, although still present, usually do not present a problem. In a known filter network, undesired harmonics are suppressed by a narrow-band band pass filter formed by two or more coupled resonators. One example of such a filter is an aperture coupled helical resonator filter.
A known band pass configuration coupling network is shown in U.S. Pat. No. 2,303,410--Toth (Dec. 1, 1942), the disclosure of which is incorporated herein by reference. FIG. 1 (prior art) of this application substantially reproduces FIG. 3 of that patent. Although the patent shows a coupling network for an intermediate frequency amplifier, it is illustrative of the typical network arrangement that is known for use in multiplier stages. Referring now to FIG. 1 (prior art), the known filter network includes a first resonant circuit including fixed inductors L1 and L3 and a variable capacitor C1. This first resonant circuit is coupled via a coupling capacitor C5 to a second resonant circuit. The second resonant circuit includes a fixed capacitor C4, a fixed inductor L2 and a variable capacitor C2. Inductor L1 and capacitor C1 form a rejection circuit for a frequency above the intended passband (a desired harmonic) in a multiplier for a frequency below the desired passband (in the case of a multiplier, an undesired harmonic or a fundamental frequency). Inductor L3 resonates with the equivalent capacitive reactance of the L1-C1 combination at the passaband frequency. Capacitor C4 resonates with the equivalent inductive reactance of the L2-C2 combination at the passband frequency.
In essence, the Toth arrangement provides a coupling network that generates rejection notches above and below the passband of the network. However there is an operational disadvantage associated with this type of network. It is substantially a fixed network in which component values must be appropriately selected. These networks are not tunable over a frequency range but will only operate at a fixed design frequency. The variable capacitor and inductor merely serve to peak their respective rejection circuits at the appropriate "design" frequency. This known network cannot be tuned to operate over a predetermined frequency range. It merely operates at a single frequency. Ideally, the network would be able to tune to a given harmonic of a fundamental frequency. When that fundamental frequency changes within a range, the circuit can be easily tuned to the given harmonic of the new fundamental frequency. In tuning to a new frequency, the relative locations of the rejection notches would be maintained at a fixed ratio with respect to the center tuned frequency.
Furthermore, for sufficient rejection of adjacent harmonics with an acceptable insertion loss, high-Q resonators are required. Due to their high cost and large size, it is impractical to incorporate high-Q resonators into miniaturized personal radios such as the PI-filters currently being used.
Since, as a design criterion, attenuation needs to be achieved only at harmonic frequencies, a matching network with finite attenuation poles at these frequencies would appear to be a promising design alternative. However, it is essential that when such a network is tuned to cover a band, i.e. capable of being tuned over a range of frequencies, that the rejection notches also move and that they remain in their proper relative locations. It would therefore be desirable to be able to accomplish tuning with a minimum of interacting adjustments. An ideal network would require only the peaking of stages to obtain both the band pass tuning for the desired harmonic and the tuning of rejection traps at the appropriate frequencies.